Discrete electronic states lying above the first ionization threshold are energetically degenerate with states that are members of a continuous spectrum. Such discrete continuum states were postulated in the early days of quantum mechanics by figures such as von Neumann and Wigner [1]}, and were later studied by Fano [2]}. The configuration interaction between the discrete and continuous spectra causes such continuum-embedded discrete states to become quasi-bound and thus possess a finite lifetime. The coupling between the resonances and continuum creates a substantial challenge for theoretical descriptions of electronic structure, and on detailed experimental measurements to test those methods.
